This invention relates to a bolometer material and a bolometer thin film that are suitable for use in a non-cooled infrared sensing element which reads signals on radiation intensity of infrared radiation by the use of a material that absorbs incident infrared radiation, changes its temperature to thereby change its electrical resistance with the temperature change, as well as to a method for producing a bolometer thin film and an infrared sensing element utilizing these techniques.
In typical bolometer-type infrared sensing elements, a photoreceptor absorbs incident infrared radiation to thereby change its temperature, the temperature change of the photoreceptor then causes a change in electrical resistance of a material in the photoreceptor, and the radiation intensity of the infrared radiation is detected as an electrical signal based on the change in electrical resistance. The sensitivity of detection increases with an increasing temperature-dependency (temperature coefficient of resistance; TCR) of the electrical resistance change. Thin films for use in bolometer-type non-cooled infrared sensing elements are thin films which absorb infrared radiation at room temperature to change their temperatures to thereby change their electrical resistance. As such thin films, thin films made of silicon (Si), germanium (Ge), or divanadium trioxide (V2O3) used as semiconductor materials have been used in practice. Silicon (Si) has a |TCR| of about 1.5%/deg., and the divanadium trioxide (V2O3) thin film having a relatively high sensitivity has a |TCR| at room temperature of about 2.0%/deg.
To further improve the sensitivity of such non-cooling infrared sensing elements, an infrared sensing element using a bolometer having a temperature coefficient of resistance (|TCR|) at 20xc2x0 C. of 0.4 to 3.9%/K has been proposed. The bolometer material for use in the infrared sensing element in question is a semiconductor including a crystalline or polycrystalline oxide or fine-structured amorphous substance comprising (1) one or more elements selected from barium, strontium, and calcium, (2) one or more elements selected from yttrium, lanthanum, and rare earth elements, (3) copper, and (4) oxygen. More specifically, the material is a yttrium (Y)-barium (Ba)-copper (Cu)-oxygen (O) system (hereinafter briefly referred to as xe2x80x9cYBCOxe2x80x9d) material having a compositional ratio of Y:Ba:Cu:O of 1:1.2-2.1:3:7-9 (Japanese Unexamined Patent Application Publication No. 11-500578).
FIG. 4(A) illustrates an example of a configuration of an infrared sensing element using such a conventional YBCO thin film as described in Japanese Unexamined Patent Application Publication No. 11-500578 as a bolometer thin film. FIG. 4(A) is a cross sectional view of the infrared sensing element. FIG. 4(A) shows that the conventional infrared sensing element comprises a silicon (Si) substrate 100 and a monolithic transducer structure 108 floated over a cavity 107. FIG. 4(B) is a top view of the silicon substrate 100. In FIG. 4(B), the top of the cavity 107 is almost continuously formed along the outer periphery of the monolithic transducer structure 108 except for part of the outer periphery. A portion remaining after etching serves as a support 109 and fixes the monolithic transducer structure 108 to the silicon substrate 100. The monolithic transducer structure 108 has a multilayer structure as shown in FIG. 4(A). Specifically, the monolithic transducer structure 108 comprises a first silicon nitride layer 101, a first yttrium-stabilized zirconia layer 102, a YBCO layer 103, a second yttrium-stabilized zirconia layer 104, and a second silicon nitride layer 105. A metallic lead 106 is electrically connected with the YBCO layer 103. The second silicon nitride layer 105 plays roles as a protecting film and as the support 109.
The sensitivity of such infrared sensing elements is proportional to the temperature coefficient of resistance (TCR) and is inversely proportional to voltage noise induced by various causes occurring upon the manufacture of the infrared sensing elements. The resistance increases or decreases with temperature change, but the temperature coefficient of resistance is evaluated on the basis of the absolute value of the change of the resistance and is hereinafter referred to as xe2x80x9c|TCR|xe2x80x9d.
To increase the sensitivity of non-cooled infrared sensing elements, |TCR| at room temperature should be equal to or more than 2.5%/deg. and should preferably be equal to or more than 3.0%/deg. As is described above, conventional silicon (Si), germanium (Ge) or divanadium trioxide (V2O3) thin films each have |TCR| less than 2.5%/deg., and resulting infrared sensing elements cannot have high sensitivity, as long as they include these thin films as a bolometer.
The YBCO thin film has a high |TCR| at room temperature, exhibits a low noise voltage and is a promising bolometer thin film to increase the sensitivity of infrared sensing elements. However, YBCO materials are apt to absorb moisture and carbon dioxide gas upon manufacture and, when they are used as a sputtering target, the resulting target splits in some cases. In addition, the YBCO thin film exhibits an increased electric resistance when it is left in the air, thus causing variations and deterioration upon handling.
The present invention has been accomplished to solve these problems, and it is an object of the present invention to provide a bolometer material having a high temperature coefficient of resistance (|TCR|), to provide a bolometer thin film exhibiting low voltage noise and a small variation in electric resistance and a method for producing the same, and to provide a highly sensitive infrared sensing element using the bolometer thin film.
To achieve the above objects, the present invention provides a bolometer material mainly comprising an oxide represented by formula: ZyCuOx, wherein Z is one or more of alkaline earth metals, one or more of rare earth elements selected from yttrium and lanthanoid elements, one or more of elements belonging to Period 5 or Period 6 of the Periodic Table selected from bismuth, lead, thallium, mercury and cadmium, or potassium or sodium; y is a number satisfying the following condition: 0 less than yxe2x89xa62; and x is a number satisfying the following condition: 0.5yxe2x89xa6xxe2x89xa61.5+2y.
The bolometer material of the present invention may mainly comprise CuOx wherein y in the elemental compositional formula equals zero.
In the bolometer material of the present invention just mentioned above, x in CuOx may satisfy the following condition: 0.5 less than x less than 1.2
In the bolometer material of the present invention, x in CuOx may satisfy the following condition: 0.6xe2x89xa6x less than 1.0
In the bolometer material of the present invention, Z may be an element selected from alkaline earth metals.
In the bolometer material of the present invention, it is preferred that Z is an alkaline earth metal, y satisfies the following condition: 0 less than y less than 1.2, and x satisfies the following condition: y less than xxe2x89xa61.5+y
In the bolometer material of the present invention, Z may be a rare earth element selected from yttrium and lanthanoid elements.
Z in the bolometer material of the present invention may be an element belonging to Period 5 or Period 6 of the Periodic Table selected from bismuth, lead, thallium, mercury and cadmium.
Z in the bolometer material of the present invention may also be an alkali metal selected from potassium and sodium.
In the bolometer material, Z in the elemental compositional formula may be a rare earth element selected from yttrium and lanthanoid elements, and the bolometer material may further comprise an oxide of an alkaline earth metal selected from beryllium and magnesium.
The bolometer material mainly comprising the oxide according to the invention may further comprise lithium or gold as an accessory component.
The present invention also provides a bolometer thin film mainly containing the bolometer material mainly comprising any of the oxides according to the invention and having an amorphous structure as its crystal structure.
In addition, the present invention provides a method for producing a bolometer thin film. The method includes the step of forming a bolometer thin film, the bolometer thin film mainly comprising an oxide represented by formula: ZyCuOx, wherein Z is one or more of alkaline earth metals, one or more of rare earth elements selected from yttrium and lanthanoid elements, one or more of elements belonging to Period 5 or Period 6 of the Periodic Table selected from bismuth, lead, thallium, mercury and cadmium, or potassium or sodium; y is a number satisfying the following condition: 0xe2x89xa6yxe2x89xa62; and x is a number satisfying the following condition: 0.5y less than xxe2x89xa61.5+2y, by sputtering using a mixture of an oxide of Z and an oxide of Cu or a complex oxide of these elements as a target.
The method for producing a bolometer thin film of the invention may include the step of forming a bolometer thin film mainly comprising CuOx, wherein 0.5 less than x less than 1.2, by sputtering using Cu2O, CuO, or a mixture of Cu2O and CuO as a target.
In addition and advantageously, the present invention provides a bolometer-type infrared sensing element for electrically detecting a temperature change induced by absorption of infrared radiation, including:
a silicon substrate;
a silicon dioxide layer lying over the silicon substrate;
a wiring layer lying over part of the silicon dioxide layer;
an electrode layer lying over part of the wiring layer and part of the silicon dioxide layer; and
a bolometer thin film lying over part of the electrode layer and part of the silicon dioxide layer, which bolometer thin film mainly comprises the bolometer material mainly containing any of the oxides according to the present invention and has an amorphous structure as its crystal structure.
In the infrared sensing element, the electrode layer may be patterned in a multi-fingered form.
In this connection, in oxides represented by xe2x80x9cZyCuOxxe2x80x9d for use in the bolometer material and others of the present invention, Z is one or more of alkaline earth metals, one or more of rare earth elements selected from yttrium and lanthanoid elements, one or more of elements belonging to Period 5 or Period 6 of the Periodic Table selected from bismuth, lead, thallium, mercury and cadmium, or potassium or sodium. A mixture of two or more types of elements can be used as Z in respective groups of the alkaline earth metals, of rare earth elements and of elements belonging to Period 5 or Period 6 of the Periodic Table for the similarity of the elements belonging to one group. However, a mixture of two or more types of elements belonging to two or more of these groups cannot be used to achieve the objects of the present invention.
The oxide represented by xe2x80x9cZyCuOxxe2x80x9d for use in the present invention can be a simple mixture, a complex oxide, or a compound of the elements represented by Z and a copper oxide and may exist as a crystal, an amorphous substance or a mixture of a crystal and an amorphous substance depending on, for example, the properties, size and electron orbit of the element Z. Among them, the oxide is preferably amorphous for higher sensitivity for infrared radiation and less noise.